But a remote sync option like CloudKit is not the panacea you’re making it out to be. I can drive five minutes from this location and have essentially no Internet connectivity. Even if ubiquitous, the cloud is not omnipresent and omnipersistent. These are things you should keep in mind.
I can report that the Private Internet Access VPN does support synching DT databases between iPhone & Mac over Bonjour, provided that, on the iPhone, Settings-Privacy-Allow access to devices on local network is enabled; and on Mac, Settings-Network-Allow LAN traffic is enabled. Perhaps other VPNs have similarly-named switches.
I’d expect that to work in general but have never tried it. VPNs build a virtual network, so all devices connected to the same VPN are on the same network.
The problem seems to be that the multicast announces are not sent over VPN (in general), so device discovery is not possible.
I found several explanations on the Net describing how to overcome that issues, but most of them seem to be in German.